1. Field of the Invention
The present invention relates to catalysts. In particular, the present invention relates to catalysts that can be used for the production of hydrogen from hydrocarbon fuels.
2. Discussion of the Background
Hydrogen production from natural gas, propane, liquefied petroleum gas, alcohols, naphtha and other hydrocarbon fuels is an important industrial activity. Hydrogen is used industrially in the metals processing industry, in semiconductor manufacture, in petroleum desulfurization, for power generation via electrochemical fuel cells and combustion engines, and as a feedstock in ammonia synthesis and other chemical processes.
Hydrogen is typically produced industrially from hydrocarbon fuels via chemical reforming using combinations of steam reforming and partial oxidation. Steam reforming of the simple hydrocarbon methane occurs via the following reaction:CH4+H2O→CO+3H2 This reaction occurs in the presence of a catalyst and is highly endothermic. The extent of the reaction is low at low temperatures. In conventional reforming processes, a temperature as high as 800° C. is often required to convert an acceptable amount of hydrocarbon fuel into carbon monoxide and hydrogen.
The steam reforming catalyst typically employed in industrial reactors contains an active Ni metal component supported on a ceramic oxide containing a mixture of aluminum oxide with Ca or Mg. However, O2 present in hydrocarbon fuel can cause the Ni to form nickel oxide, which is inactive as a steam reforming catalyst. The Ni metal can also react with the aluminum oxide of the support to form compounds that are catalytically inactive for steam reforming, such as nickel aluminate spinel. This detrimental interaction between active metal and support can significantly reduce catalyst activity over long periods of operation.
In some cases reforming catalyst is exposed to cyclic operation conditions of reactor shut-downs and restarts. This cyclic operation is more important for fuel cell and small scale hydrogen generation plants than for conventional large scale hydrogen production plants. During reactor shut-down, it is desirable that exposure of catalyst to air does not lead to a significant loss in catalytic activity. However, exposure of Ni to air during each cycle incrementally leads to reduced catalyst activity as the Ni becomes increasingly oxidized. Under these conditions, the oxidized nickel must be reduced if the Ni-based catalyst is to regain activity.
Because O2 may be present at relatively high levels in hydrocarbon feeds, especially in natural gas obtained from a utility, a process for removing O2 from the hydrocarbon must be included upstream of the reforming reactor to avoid oxidation of the Ni metal catalyst.
An additional problem with conventional Ni-based catalyst is that the Ni metal is susceptible to poisoning and deactivation by trace levels (˜1 ppm) of sulfur (S) in the reacting hydrocarbon fluid. Removal of sulfur to levels acceptable for Ni-based reforming catalysts requires a hydrodesulfurization process and a sulfur absorption bed, both of which add to the complexity, cost and size of the reformer system.
Alternative catalysts for steam reforming processes have been proposed.
Rostrup-Nielsen, Jens R., Catalytic Steam Reforming, Springer-Verlag, Berlin, 1984, suggests that for steam reforming Rh and Ru are the most active catalysts, while Pt, Ni and Pd are all comparable, and Ir is less desirable.
U.S. Pat. No. 4,988,661 discloses hydrocarbon steam reforming catalysts having nickel oxide, cobalt oxide and/or platinum group noble metals supported on a carrier consisting essentially of aluminum oxide and an oxide of Ca, Ba and/or Sr.
U.S. Pat. No. 6,238,816 discloses sulfur-tolerant catalysts for hydrocarbon steam reforming. The catalysts contain active metals of Ag, Co, Cr, Cu, Fe, Pd, Pt, Ru, Rh, and/or V supported on various oxide materials.
While conventional hydrocarbon steam reforming catalysts provide improved initial activity and sulfur tolerance relative to Ni-based catalysts, conventional catalysts fail to provide stable performance over extended periods of time upon exposure to both air and reducing atmospheres. Conventionally, catalyst stability is measured in air. However, catalyst stability in air is no indication of catalyst stability in low oxygen and reducing environments.